Droplet heterogeneous nucleation in a rapid expansion aerosol chamber.

We present a new experimental facility to investigate the nucleation and growth of liquid droplets and ice particles under controlled conditions and characterize processes relevant to cloud microphysics: the rapid expansion aerosol chamber (REACh). REACh is an intermediate size chamber (∼0.14 m3) combining the principle of an expansion chamber with the ability to probe the influence of turbulent flows. Water droplet heterogeneous nucleation onto seeding aerosols is achieved via a sudden pressure drop accompanied by a temperature drop, which can cause humid air to condense into a cloud of droplets under appropriate thermodynamic conditions. REACh features tight control and monitoring of the initial saturation ratio of water vapor, identity and concentration of seeding aerosol particles, temperature, pressure, and air flow mixing, together with high speed real-time measurements of aerosol and droplet size and number. Here, we demonstrate that the minimum temperature reached during each expansion can be reasonably described by the thermodynamics of dry or moist adiabats for a range of initial relative humidities. The size and number of droplets formed and the overall lifetime of the cloud are characterized as a function of the aerosol concentration and initial water vapor saturation ratio. The total droplet concentration scales linearly with the seeding aerosol concentration, suggesting that all injected aerosol particles serve as condensation nuclei. While the total number of droplets formed increases with aerosol concentration, the mean droplet size decreases with the concentration of seeding aerosols as a result of competition for the available water vapor. Theoretical considerations provide a quantitative prediction for the mean droplet size over a range of conditions. The high repetition rate of experiments that we can perform with the REACh facility will permit extensive characterization of aerosol processes, including droplet and ice nucleation onset and growth, and the importance of turbulence fluctuations. We will leverage the capabilities of this facility to explore a wide range of physical parameters encompassing regimes relevant to cloud microphysics.